The highly polymorphic killer immunoglobulin-like receptors (KIRs) and their HLA class I ligands play a central role in the regulation of natural killer (NK) cells, and in the ability of HIV-1 infected individuals to control virus replication. However, functional studies to address the significance of KIR-MHC class I interactions in immunodeficiency virus infection have been limited by the lack of defined MHC class I ligands for KIRs in non-human primate models. We recently identified Mamu-A*02, an MHC class I molecule present in approximately 20% of Indian origin rhesus macaques, as a ligand for Mamu-KIR3DL05 (3DL05). This interaction was peptide-dependent, since Mamu-A*02 tetramers folded with certain simian immunodeficiency virus (SIV) peptides, but not others, stained primary NK cells and transfected cells expressing multiple 3DL05 alleles. Consistent with the function of an inhibitory KIR, target cells expressing Mamu-A*02 (A*02) suppressed the degranulation of tetramer-positive NK cells from 3DL05+ macaques. These observations suggest that SIV, and potentially also HIV-1, may acquire changes in epitopes that increase the avidity of MHC class I ligands for inhibitory KIRs as a mechanism of immune evasion to prevent the activation of specific NK cell subsets. Using KIR- and MHC class I-defined rhesus macaques, we will specifically address this hypothesis, as well as other questions fundamental to the biology of NK cells in immunodeficiency virus infection. The first objective of this proposal (Aim 1) is to identify additional MHC class I ligands for Mamu KIR3DL01 and -KIR3DL05; two KIRs that are commonly expressed in the rhesus macaque and for which we have identified reagents for staining these receptors on primary NK cells. This aim will build on recent work by our group to provide a broader foundation for investigating the role of KIR-MHC class I interactions in SIV infection. Our second objective (Aim 2) is to address the functional implications of viral peptides that modulate NK cell activation. We will determine the repertoire of SIV peptides that enhance or antagonize interactions with 3DL05, and will test the hypothesis that viral epitopes, which stabilize interactions with inhibitory KIRs, facilitate virus replication in the presence of NK cells bearing these receptors. Our third objective (Aim 3) is to compare longitudinal changes in 3DL05+ NK cells following SIV infection of 3DL05+/A*02+ versus 3DL05+/A*02- animals. The following questions will be addressed; Does the recognition of A*02-bound peptides stimulate or suppress the expansion of 3DL05+ NK cells? Are there phenotypic/functional differences in 3DL05+ NK cells in A*02+ versus A*02- animals? Is there a difference in the recruitment of 3DL05+ NK cells to tissues? These unprecedented studies will provide a better understanding of the importance of KIR-MHC class I interactions in immunodeficiency virus infection, and specifically the role of viral peptides in modulating NK cell activation as a mechanism of immune evasion.